If you are using KDE look down in the lower right hand corner of your task bar in the system tray. This is the "front end GUI" called Knetworkmanager which gives the user GUI control over the backend "network-manger". Right click on this icon and it will give you a pop-up box > hover on "options" or you can click "manual configuration".

Can you see the icon ok? This program runs at startup and should appear by default in the system tray.

If for some reason the icon doesnt appear there open a terminal and type: